In the fields of audio and music, it is fairly common to apply some type of dynamics processing to audio. Dynamic processing generally focuses on the volume of audio, but not necessarily in a linear fashion. In general, a dynamics processor takes an input audio signal, and produces an output audio signal with the amplitude (volume) adjusted, typically with some time-varying gain.
A common type of dynamics processing is known as compression, wherein the higher the input signal level, the higher the attenuation. Many compressors work using a threshold, in that when the input signal increases in magnitude and crosses the threshold, the attenuation increases by some ratio. Typically, compressors measure signal magnitude by RMS to get an average of the signal energy, although other magnitude measurements, such as peak value, can also be used.
One specific type of compressor is known as a limiter. Limiters typically have a goal of preventing the audio signal from going above a specific threshold, or ceiling. For digital audio signals, the ceiling set by the bit count cannot be exceeded without distorting the signal. This distortion is known as clipping, in that the signal is “clipped off” without reaching its real peak amplitude. Clipping is undesirable not only because of the distortion it causes, but also the loss in the data signal may not be recoverable.
Limiters work by attenuating the signal before it reaches a peak value. Typically limiters respond much faster than compressors, both in reacting to an increased signal (the attack), and then in returning to normal (release) after the input signal drops below the threshold. Limiters often respond to the peak value of a signal instead of the average or RMS value.
An advantage of limiters is that by preventing the signal for going above a certain threshold, the signal strength can be raised to utilize the signal bandwidth available. In audio signal, this can also be used to increase the perceived “loudness” of a signal, an effect often applied for high quality audio. Sometimes this effect is overly applied to audio and music, resulting in the music sounding “squashed”, but if done carefully, this limiting can help make the audio sound more full, without overly minimizing the dynamic range.
Limiters have a disadvantage that if applied aggressively, they can make the audio sound very bad and unnatural. Based on parameter settings, limiters can cause audible distortion and other unwanted effects, such as pumping. To be useful, such bad effects of limiters must be minimized.